“”For more than four years I have had the privilege of serving President Obama and his Administration as the Secretary of Homeland Security,” Napolitano said in a statement on Friday. “The opportunity to work with the dedicated men and women of the Department of Homeland Security, who serve on the frontlines of our nation’s efforts to protect our communities and families from harm, has been the highlight of my professional career.”

Napolitano, who has been serving in her current post since 2009, was the first woman to helm of the Department of Homeland Security. A Democrat, she previously served as governor of Arizona from 2003 to 2009 and before that was the state’s attorney general.

Napolitano notified President Obama of her decision to step down a month ago, sources say. She is planning to leave her post in early September.